This invention relates to high wet modulus fibers having enhanced wet strength and toughness properties so as to be similar in characteristics to cotton.
Some wet modulus fibers of the prior art were found to have certain shortcomings that limited their use. Many were brittle and were subject to fibrillation, and it was also found that these had low abrasion resistance and poor launderability characteristics.
Methods of producing viscose rayon staple from viscose containing a cellulose component of relatively high degree of polymerization are also known. These various known methods are conventionally referred to as "high or intermediate wet modulus fiber," and have some properties close to those of cotton when compared with conventional viscose rayon staple. However, heretofore known high wet modulus fibers still lacked one or more of the preferred properties of cotton such as high alkali resistance, high tensile tenacity and a suitable ratio of wet tenacity to conditioned tenacity. Also, heretofore known methods of production of high wet modulus fiber has been relatively low.
U.S. Pat. No. 2,732,279 of Tachikawa discloses a process for producing a rayon fiber which is stated as being comparable to cotton through the essential features of dissolving cellulose so as to permit the retention of a substantial proportion of intrinsic properties of the natural fiber and with a regeneration procedure which consists of microspinning dissolved cellulose under controlled conditions. However, the total process specifications of Tachiwaka of using a spinning bath of very low acid and low salts results in a fiber which is far from the characteristics of cotton.
U.S. Pat. No. 3,139,467 of Drisch et al carries forward the teachings of Tachiwaka et al in its recognition of the need for a high degree of polymerization (DP) in the fiber. Drisch et al further utilizes the concept of highly stretching the filaments while the fiber is still in the xanthate gel state which requires the utilization of a dilute acid spinning bath with a low salt content. Drisch et al further adds formaldehyde into the dilute bath which, as a result of crosslinking, causes the fiber to have some cotton-like properties but also causes the fiber to be very brittle, which is not at all cotton-like.
U.S. Pat. No. 3,277,226 of Bockno et al and U.S. Pat. No. 3,529,052 of Carney et al. each relate to the development of the so-called high wet modulus fibers, and for the first time there was prepared a synthetic fiber having a cotton-like wet strength, and that approached cotton in low water pick-up and low shrinkage characteristics without being excessively brittle. These fibers and processes were developments in tire cord technology which by virtue of well known viscose additives and high concentrations in the spinning bath of zinc salts yielded a very strong, tough and resilient fiber. The innovations resulted in a fiber with high wet strength, high wet modulus and low shrinkage which added to tire cord toughness and resilience. Nonetheless, these high wet modulus fibers were still somewhat deficient as compared to cotton in water stability and resistance to caustic, which is a good indicator of wet performance.
U.S. Pat. No. 3,434,913 of Bockno et al. and U.S. Pat. No. 3,494,996 of Stevens et al. relate to the preparation of viscose rayon fibers having high strength characteristics and a high wet modulus without being excessively brittle or fibrillatable. However, each of these patents following Drisch disclose the utilization of formaldehyde in the stretch bath which is now known to alter the characteristics of the fiber away from cotton, and introduce other undesirable properties.
With the advent of high wet modulus rayon fibers, toughness and particularly wet performance were substantially improved. High wet modulus rayon became reasonably competitive with cotton in factors including shrinkage, wear resistance, wet performance, and launderability. Accordingly, it was thus possible to place the high wet modulus rayon fibers in many end use fabric applications where rayon had been wholly unsatisfactory before. Examples include sheets and men's shirting fabric. In these applications, the high wet modulus rayon did approximate cotton insofar as wear, abrasion resistance, and launderability characteristics were concerned. For all practical purposes, these high wet modulus (HWM) rayon containing fabrics could withstand a number of launderings without serious deterioration of the fabric.
Nevertheless, cotton still held a real advantage over rayon in the above noted group of properties and also exhibited a decided advantage over rayon in many laboratory tests designed to simulate or predict real fabric performance.
The following Table shows the approximate ratings resulting from various laboratory evaluations of wear by various well known procedures.
TABLE ______________________________________ Reg. Rayon HWM Rayon Cotton ______________________________________ Untreated Fabric Stoll Flat Abrasion Conditioned 85 95 120 Wet 20 30 75 Stoll Flex Conditioned 110 110 200 Wet 85 200 350 Accelerator % Weight Loss Conditioned 1.7 2.0 3.0 Wet 1.5 0.2 0.4 Solubility in 10% NaOH % Dissolved 12% 8% 6% ______________________________________
One of the above properties which seems to indicate the toughness of the fiber is that of caustic solubility.
A further qualitative or semi-quantitative evaluation of wear is the appearance of fibrillations along fabric creases. Fibrillation becomes apparent in dyed fabrics and is manifested as a ligher colored fuzz on the fabric surface. This phenomenon appears long before the fabric itself shows a crease or a break. With respect to fabric fibrillation, high wet modulus rayons usually exhibit more fibrillation than do cotton fibers.
It is most important to note that in considering wear evaluation of fibers, fabrics deteriorate far more by washing or cleaning than they do by actual wearing. Accordingly, the behavior of the fiber and fabric in washing machines is more significant than what happens to the fabric while in actual use. By this modern criterion, cotton still has a small but significant advantage over the utilization of conventional rayon and high wet modulus rayon fabrics.
One additional quality which rayon showed at a disadvantage to cotton was in "cover", by which we mean that the same weight of cotton yarn seemed to occupy more volume than its equivalent rayon. By introducing a slight crimp to the rayon, and adding small quantities of delustrants to the rayon, one could make the rayon equivalent to cotton in this quality.
The above noted disadvantages of the utilization of rayon have now been overcome by the creation of a new rayon fiber which is fully equal to cotton in wet toughness, resistance properties and in cover, while maintaining all of the other desirable properties of high wet modulus rayon, namely, high strength conditions, good dyeability, high moisture regain, shrinkage resistance, superior carding properties and the superior spinning and weaving properties of high wet modulus rayon.